Tubulointerstitial renal disease is an important component of the pathobiology of HIV-associated nephropathy (HIVAN), the most common cause of chronic renal failure in HIV-infected individuals. In HIVAN, HIV infection of renal tubular epithelial cells (RTECs) leads to dysregulated apoptosis and production of proinflammatory molecules, both of which contribute importantly to progressive renal failure. HIVAN occurs almost exclusively in people of African ancestry and the genetic background of the individual is a critical determinant of whether renal epithelial infection by HIV will lead to progressive renal disease. We have recently demonstrated that the ubiquitin-like protein FAT10 is highly upregulated by HIV-infection of human RTECs in vitro and in HIVAN biopsy specimens, that FAT10 expression induces apoptosis in RTECs, and that preventing FAT10 expression with shRNA constructs ameliorates HIV-induced RTEC apoptosis. Preliminary studies in our laboratory have also demonstrated that FAT10 expression upregulates NF-KB signaling in human RTECs and may therefore induce production of proinflammatory mediators by these cells. We have identified four alleles of the FAT10 gene, one of which is significantly more common in patients with HIVAN. Since RTEC apoptosis and production of proinflammatory mediators by RTECs are important factors in HIVAN pathogenesis, we hypothesize that polymorphisms in the FAT10 gene alter its ability to induce RTEC apoptosis and NF-kB activation in RTECs. We will test these hypotheses in the following three Specific Aims: 1: To determine the effects of nonsynonymous FAT10 SNPs in FAT10 upon its subcellular localization and proapoptotic function. In this aim, we will determine how the four variants of the FAT10 protein differ in their subcellular localization and their ability to induce apoptosis. 2: To define the role of FAT10 in NF-KB signaling in human renal tubular epithelial cells and to determine if the FAT10 alleles differ in their ability to stimulate NF-KB signaling. In aim 2, we will study the mechanisms by FAT10 activates NF-KB signaling. 3: To determine whether FAT10 expression is necessary for the development of the HIVAN phenotype in the HIV-1 transgenic model of HIVAN. In aim 3, we will breed FAT10 knockout mice with HIV-1 transgenic mice and study the effect of FAT gene deletion upon the HIVAN phenotype. These studies will elucidate novel mechanisms of disease progression in HIVAN.